[Feature] Normalize

[zicklag]

Hey there, very cool tool. 😃 👍 A feature I use from imagemagic that imagecli doesn't have yet is the normalize command. I use it to get the best contrast out of my printer scans without having to even open an image editor or tweak any settings.

[theotherphil]

This is just a special case of contrast stretching which imageproc already supports for grayscale images (and so would be easy to add support for here). We'd need to update imageproc to support colour images.